Mantle flow and lithosphere–asthenosphere coupling beneath the southwestern edge of the North American craton: Constraints from shear-wave splitting measurements

High-quality broadband seismic data recorded by the USArray and other stations in the southwestern United States provide a unique opportunity to test different models of anisotropy-forming mechanisms in the vicinity of a cratonic margin. Systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations. The western and southern edges of the North American craton show edge-parallel fast directions with larger-than-normal splitting times, and the continental interior is characterized by smaller splitting times and spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of North America. At the majority of the stations, no significant systematic azimuthal variations of the splitting parameters are observed, suggesting that a single layer of anisotropy with a horizontal axis of symmetry can adequately explain most of the observations. The spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere. Based on previous results of seismic tomography and geodynamic modeling, we propose a model involving deflecting of asthenospheric flow by the cratonic root as the cause of the observed edge-parallel fast directions and large splitting times along the western and southern edges of the North American craton.